[RemoveDIs][DebugInfo] Explicitly convert iterator to pointer for std::distance

This is a small patch attempting to fix an error with a prior patch which
caused buildbot failures on certain targets; for example:
https://lab.llvm.org/buildbot/#/builders/127/builds/60931

The compile error comes from std::distance not having an overload for a
std::array iterator and a raw pointer; this patch converts the iterator
to a pointer to resolve this.

Fixes d499df02